The present invention relates to a smoke or fire detector that first utilizes the initial signal that is indicative of a detected smoke or fire condition to operate an air moving fan to purge the detector's photocell or ionization chamber insted of sounding an alarm. Thereafter, any additional signal indicative of a detected smoke or fire condition within a predetermined time frame is utilized to signal an alarm. The detector's photocell or ionization chamber detects smoke or a fire condition after an initial purge cycle.
In the past elaborate systems have been devised to prevent false alarms from fire and smoke detectors and yet to continue to detect fire and smoke. Sensitivity compensated fire detectors such as disclosed in Ray W. Klein's U.S. Pat. No. 4,097,851 have been disclosed. Other alarm devices have provided reference and measurement chambers as disclosed in the Emele L. Hugon U.S. Pat. No. 4,238,677. Other systems have been proposed and tried.
Prior fire and smoke detectors have employed a sensing device, such as a photocell or more commonly an ionization chamber, for detecting products of combustion. In an ionization chamber, a source of radiation ionizes air molecules between a pair of spaced electrodes, across which a voltage is applied to establish an electric field. The electric field moves the ionized molecules between the electrodes to provide a current flow. The space between the electrodes is open to the atmosphere, and upon the occurrence of fire products of combustion enter the chamber and combine with ionized air molecules to reduce the current flow and thereby increase the impedance of the chamber. This change in impendance occurs relatively rapidly and generates an electric signal indicative of a detected smoke or condition which, through detecting circuitry, causes an alarm to be sounded.
A disadvantage of such detectors is that certain naturally occurring atmospheric conditions, such as changes in atmospheric pressure and relative humidity, accumulation of films of dirt and dust, etc., affect the magnitude of the current flow in the detecting chamber and may change the sensitivity of the detector, rendering the detector in many cases susceptible to generating spurious alarms.
One way that has been disclosed to minimize the occurrence of spurious alarms is to decrease the sensitivity of the detector to account for all reasonably anticipated variations in ambient conditions. Unfortunately, this technique also reduces sensitivity to products of combustion.
Other prior art devices include detectors having a sensor device for detecting a predetermined phenomenon, and in particular to a detector which is compensated against changes in sensitivity resulting from natural and slowly occurring ambient variations which affect the sensor.
Others have provided devices that deal with smoke and fire detectors of the type in which detection is performed by measuring ionization of the ambient environment, and deals more particularly with a detector provided with an electronic velocimetric circuit which screens out spurious, false alarm signals produced by pollutants within the ambient environment, inadvertant short term connecting or disconnecting of the detector, electrical interference signals, faulty insulation of electrical components, variations in supply voltage, and the like.